Block copolymers of styrene and conjugated diolefins have long been known as useful as elastomeric thermoplastics. These block copolymers are referred to as thermoplastic polymers because they may be worked by heating the polymer to above the polymer's melting temperature, and then processing by such methods as vacuum forming, extrusion, compression molding, calendaring or blow molding. These block copolymers have good tear resistance, flexibility, thermochemical stability and other properties.
The thermoplastic properties of block copolymers of styrene and conjugated diolefins are the result of incompatibility between the polystyrene and the polydiolefin polymeric blocks which causes separate polymeric phases to exist. At service temperatures, the polydiolefin domains are rubbery and elastic, whereas the polystyrene domains are hard and glassy. The polystyrene domains serve as physical crosslinks between the rubbery polydiolefin blocks. This causes the polymer to behave much like a vulcanized rubber at temperatures that are below the polystyrene glass transition temperature. By heating the block copolymer to a temperature higher than the glass transition temperature of the polystyrene domains, the polymer may be processed as a melt and formed into useful shapes.
Blown films of about 1 mil thick have not been commercially made from block copolymers of styrene and conjugated diolefins because blown films using typical blown film extruders and dies have resulted in films of about two mils thickness or greater. Films of this thickness can be prepared by other processes that are simpler than blown film technology. It would be desirable to have thinner blown films for many different uses, such as elastic gloves and baby diaper waist bands. Blown films of other materials can be made of one mil thickness or less, but the elastomeric copolymers of block copolymers of styrene and conjugated diolefins have properties that make blown film extrusion very difficult. Previous attempts to blow-extrude films of these copolymers to thicknesses of about one mil have been unsuccessful.
It is therefore an objective of the present invention to provide a method to prepare blown films of block copolymers of vinyl aromatics conjugated diolefins wherein the blown films have a thickness of less than about one mil. It is further object to provide such a method wherein commercially available blown film extruders can be used without significant modifications.